The Problem: Some Objects Are Simply Too Big
Every telescope and camera combination paints a fixed window on the sky, and some of the best deep sky objects refuse to fit inside it. That is exactly what happened with the Heart and Soul Nebulae in Cassiopeia. Both were captured here with an Orion 8 inch Newtonian reflector at 1000mm f4.9 and a Canon T3i camera, a combination that frames roughly 1.3 by 0.85 degrees of sky. The Soul Nebula complex covers an area equal to about four full moons. One frame simply cannot hold it.
The fix is a mosaic: shoot several overlapping frames that tile the target, then stitch them into a single image during processing. The idea is simple. The execution has real science behind it, and the difference between a sloppy mosaic and a clean one is measured in the details.
How Big Is Your Window on the Sky?
Field of view follows a simple relationship: divide the sensor dimension in millimeters by the focal length in millimeters, then multiply by 57.3 to get degrees. The Canon T3i sensor measures about 22.3 by 14.9mm. At 1000mm that works out to about 1.3 degrees along the long side and 0.85 degrees on the short side.
Now compare that to real targets. The full moon spans about half a degree, so the frame comfortably holds it. The Andromeda Galaxy stretches about three degrees along its long axis, which overflows this frame even in landscape orientation. The Soul Nebula complex is similar. When your target is bigger than your window, you have two options: shorten the focal length, or start planning panels.
Overlap Is Everything
A mosaic is only as good as its seams. Neighboring panels must share enough sky for software to find matching stars and blend the pieces, and the standard rule is 20 to 30 percent overlap between adjacent frames. Less than that and star matching gets fragile. More than that and you waste precious imaging time on sky you already captured.
The numbers from this gallery show the principle at work. The Heart Nebula mosaic below is 4973 by 5757 pixels from two vertically stacked panels, with about a third of each panel height shared between the two. The Soul Nebula mosaic is 9039 by 5976 pixels from four panels arranged in a two by two grid, with roughly a quarter of each panel overlapping its neighbors.
Overlap also saves you at the edges. Optical vignetting and field curvature degrade star shapes near frame borders. The overlap zone gives you the option to crop away the worst of it before the panels meet.
Planning the Grid in NINA
NINA makes mosaic planning painless. The Framing Assistant lets you define a grid of panels over any target, set the overlap percentage, and rotate the entire grid to match the natural axis of the object. That last part matters more than beginners expect. Rotate the grid so the nebula flows across panel boundaries through dense regions rather than slicing a faint outer edge in half, because faint edges are where gradient mismatches show up worst.
If you want the full walkthrough, the Framing Assistant basics post covers the fundamentals, and the deep dive on rotating, cropping, and planning mosaics goes into grid strategy in detail.
Capture Discipline
Consistency is the rule during capture. Every panel should use identical exposure length, ISO, and white balance, because any mismatch becomes a visible brightness or color step at the seams after stitching. Refocus on each panel. Star sizes that differ between panels are nearly impossible to blend invisibly. Keep dithering enabled throughout so fixed pattern noise averages out evenly across the whole mosaic.
Be honest about the time budget before you start. Each panel of the Heart Nebula mosaic in this gallery received 85 exposures of 180 seconds, which is about four and a quarter hours per panel, and the full two panel image took five nights. The Soul Nebula mosaic needed the same per panel investment across four panels. Mosaics multiply total shutter time by the panel count, so a target that needs one great night as a single frame needs two nights as a two panel mosaic and four as a grid.
Stack First, Stitch Second
The processing order matters. Stack each panel independently in Siril first, exactly as you would a normal image, including calibration, registration, and integration. Run background extraction on each stacked panel before any stitching happens, because light pollution gradients differ between panels captured on different nights, and mismatched gradients are the number one giveaway of a mosaic.
Only then load the stacked panels into Siril mosaic registration, which finds matching stars in the overlap zones, computes the offsets, and blends the panels into one image. Finish with a crop to remove the ragged outer edge where panels do not fully cover. The general Siril workflow is covered in the stacking to final image guide.
The Science Payoff
Why go to all this trouble instead of just using a shorter telescope? Resolution. Halving focal length doubles the field of view, but it also spreads the same pixels across four times the sky area, so every detail shrinks to half size. A mosaic keeps the image scale of the long focal length while expanding the field of view to whatever the target demands. The Soul Nebula mosaic in this gallery is 9039 pixels wide. Captured at half the focal length in a single frame, the nebula would occupy barely half that pixel count across.
The trade is time rather than equipment. You are choosing to spend four clear nights instead of one to hold onto fine structure. For emission nebulae with intricate filament detail, that trade is usually worth it.
Two Mosaics from This Gallery
The Heart Nebula, also designated IC 1805, is an emission nebula about 7500 light years away in the Perseus Arm of our galaxy. This two panel mosaic took five nights with the Orion 8 inch Newtonian at 1000mm, guided with a Meade 800mm guide scope on an Orion Atlas EQ-G mount. It first appeared on the blog in the Heart Nebula feature post.
The Soul Nebula, designated IC 1848 and known to radio astronomers as Westerhout 5, sits just east of the Heart and hosts embedded clusters including Collinder 33 and Collinder 34. It spans an area of sky equivalent to four full moons and sits about 6500 light years away. Four panels of 85 by 180 second exposures were stitched in Siril to hold it all.
Both images, along with the full capture details, live in the astrophotography gallery.
Zoom Out or Stitch?
So how do you decide? A quick guide:
- Stitch a mosaic when the target overflows your frame and you want to keep fine resolution, especially emission nebulae with filament structure.
- Use a shorter focal length when the target fits comfortably and you value speed, or when seeing and guiding limit your real resolution anyway.
- Use a focal reducer as the middle path, buying field of view at a modest cost in image scale.
- Skip mosaics on marginal nights. Consistent conditions between panels produce the cleanest seams, so a single excellent night beats two mediocre ones.
Try It This Season
Late October is prime time for this. Cassiopeia rides high in the evening sky, putting the Heart and Soul region near the zenith for northern observers, right where the atmosphere is thinnest and guiding is most stable. Start modest with two panels of the Heart, keep your overlap near a third, and hold every capture setting constant between panels. When the seams disappear and the whole nebula lands in one frame, you will understand why mosaic shooters accept the extra nights. The sky’s biggest objects give no other way to have them whole.
